Low loss microwave transmission lines across cryogenic temperature barriers



J. KLIPHUIS 3,389,352 LOW LOSS MICROWAVE TRANSMISSION LINES ACROSS June18, 1968 CRYOGENIC TEMPERATURE BARRIERS Filed Feb. 7, 1966 ROOMTEMPERATURE CRYOGENIC INVENTOR. JANS KLIPHU IS AT TOIRNE YS United"States Patent Filed Feb. 7, 1966, Ser. No. 525,770 6 Claims. (Cl.333-98) This invention pertains to microwave transmission lines forcoupling microwave energy across a thermal barrier. More specifically,this invention relates to a transmission line particularly adapted tocouple RF energ to or from a cryogenically cooled device in such a wayas to improve the thermal isolation between such cryogenically cooleddevice and its environment.

Gryogenically cooled devices today find numerous applications atmicrowave frequencies where, as a practical matter, waveguides must beused for the transmission of the' RF energy. Since waveguide consists ofa continuous electrical conductor such as copper or like materials,which are 'also good conductors of heat, the coupling of RF energythrough waveguide to a cryogenically cooled device presents seriousthermal insulation problems. Previousetforts to reduce the heat loss dueto the conductive waveguide sections have not been totally successfuldue primarily to the noise and/or losses contributed by the requiredstructure. For example, a long loop of waveguide has: been employed toincrease the thermal conduction path length.

Accordingly, an object of the present invention is to provide atransmission line for coupling RF energy across a thermal barrier.

Another object of the invention is to provide a microwave transmissionline for coupling RF energy to or from a cryogenically cooled devicewith improved thermal insulation and a low added noise contribution.

Briefly, in accordance with the invention, the above objectives areobtained through the use of a choke joint having a smooth flange and achoke plate containing a conyentional quarter wave groove. The flangeand choke plate are physically separated in a poor heat conducting spaceto provide thermal isolation whereby no conduction heat losses occurthrough the waveguide itself. When used with a cryogenically cooleddevice, such device is preferably coupled directly to the choke plate sothat the insertion loss caused by the choke joint will provide only aslight noise contribution due to the reduced temperature. of the plate.Should a hermetically sealed environment be necessary, the portion ofthe choke joint (generally the flange in the case of a cryogenicallycooled device) leading to the outer environment may readily be sealed ina conventional manner.

The manner in which the above and other objects of the invention areaccomplished is more fully described below with reference to theattached drawings, in which,

FIGURE 1 is a side view in section illustrating a preferredembodiment ofthe invention for use with a cryogenically cooled device;

FIGURE 2 is a view along the line 22 of FIGURE 1; and

FIGURE 3 is a view along the line 3-3 of FIGURE 1.

In FIGURE 1, block represents a cryogenically cooled device which, forexample, may be a parametric amplifier. The cryogenically cooled device10 is housed in a conventional Dewar vessel 12 and the requiredrefrigerants are provided through line 14. The construction as so farexplained, and numerous equivalents thereof, are well known and detaileddescription thereof is unnecessary. Customarily, the Dewar vessel 12 ishermetically sealed and evacuated to thermally isolate the device 10from its environment.

3,389,352 Patented June 18, 1968 ice The problem is to provide atransmission line for coupling microwave energy (e.g. the pump power)between cryogenic device 10 and a source outside Dewar vessel 12 whichisat room temperature. In the following description only a singletransmission line is shown although obviously any number may beemployed. Furthermore, no eiforthas been made to draw the parts toactual scale since the various dimensions will be appropriate fordifferent purposes as will be obvious to those skilled in the art.

According to the invention, the transmission line includes a firstwaveguide section 16 connected in a suitable manner to the cryogenicallycooled device 10, a choke joint 18, and a second waveguide section 20coupled, for example, to a source of RF energy (not shown) at roomtemperature.

Choke joint 18 is conventional and includes a choke plate 22 suitablysecured to waveguide section 16 and containing a groove 24 which isapproximately one-quarter wave length deep and located one-quarter wavelength (average distance) from the inside walls of the waveguide. Choke18 further includes a flange 28 secured to waveguide section 20 andphysically separated. from the choke plate 22 by a distance d. The face29 of flange 28 may be polished to reduce RF losses to a minimum. Oneend of waveguide 20 may include a collar 30 which contains aconventional waveguide window 32 made of glass, quartz, or otherdielectric hermetically sealed to guide 20 so as to maintain the vacuumwithin Dewar vessel 12. Such a window (not shown) may optionally beplaced in waveguide section 16 also. The junction of guide 20 and Dewar12 may be hermetically sealed by any conventional sealing means such asshown at 33. By way of example, the waveguides and choke joint may be.made of oxygen free copper.

Electrically, the operation of the device is known since the choke joint18 serves in a conventional fashion to couple the RF energy from theinput source to the cryogenically cooled device 10. Because of thephysical spacing between the choke plate 22 and the flange 28, the twosides of the transmission line are thermally insulated from each other,thereby eliminating all conduction heat losses. In accordance withconventional theory, the distance d should be less than one-quarter wavelength of the transmitted frequency, but otherwise is not critical andmay be optimized for each case with a view toward the electrical andthermal characteristics of the device.

To avoid undesired effects due to the leakage of RF energy through thegap in the choke joint, a ring of RF energy absorbent material 34 may beplaced around the circumference of the choke plate 22. This absorbentmaterial may comprise an epoxy material filled with iron powder of thetype manufactured by Emerson and Cummings, Inc., and sold under thetrademark Eccosorb.

Generally, with the choke plate 22 and flange 28 at the sametemperature, and assuming an insertion loss of about 0.2 db due to thechoke joint, the system noise temperature would be increased byapproximately 16 K. However, since most of the losses will occur at thechoke plate 22 which is thermally connected to the cryogenically cooleddevice 10, the noise contribution of the choke joint will be reducedsubstantially in proportion to the temperature reduction. For example,if the cryogenically cooled device has an operating temperature of 30K., the noise contribution of the choke joint will be less than 1.6 K.Similarly, absorption of the leakage RF energy by the ring 34 tends tominimize the noise contribution.

In addition to reducing heat losses (and thus the heat load on therefrigerator) to radiation losses alone the illustrated embodiment ofthe invention provides a number of other significant advantages. Forone, it simplifies assembly of the device 10 with the refrigeratingapparatus. For another, the distance between the Dewar wall and thedevice 10 is not relied upon for thermal isolation and can be reduced toless than one wave length thereby reducing the Dewar size, and thevacuum retention problem. Third, the noise temperature of the completesystem is reduced substantially over those using prior thermal isolationtechniques.

It will be apparent to those skilled in the art that the invention isnot necessarily limited to cyrogenic applications. Thus, the inventionwould have equal utility in coupling energy between any two deviceswherein a substantial temperature differential is to be maintained.Also, the invention would have utility in coupling energy to or from adevice which is to be maintained at a particular temperature withrespect to an environment which is subject to temperature fluctuations.In those cases where one of the two devices is to be at a temperaturesubstantially lower than the other, the choke plate should preferably beconnected to the device maintained at the lower temperature to reducethe noise contribution. Otherwise, the relaitve position or arrangementof the choke plate and flange is not material.

What is claimed is:

1. A device for coupling microwave energy between a first device at onetemperature and a second device at a temperature substantially differentfrom said one temperature, comprising means for thermally insulatingsaid second device from said first device, a first waveguide section ina thermally conductive relationship with said first device, a secondwaveguide section adapted to couple energy to or from said seconddevice, said first and second waveguide sections being spaced apart soas to interrupt the heat conduction path therebetween, and choke meanspositioned at the gap between said waveguide sections for reducing theamount of microwave energy radiated from said gap.

2. Apparatus for coupling microwave frequency energy between a firstdevice at one temperature and a second device at a substantiallydilferent temperature, comprising insulating means including anevacuated chamber substantially surrounding said second device, a firstwaveguide section coupled to said first device and extending into saidevacuated chamber, a second waveguide section coupled to said seconddevice and extending into said evacuated chamber with a substantial gapbetween said first and second waveguides not greater than approximatelyone-quarter wavelength of said microwave frequency, and choke meanspositioned on respective sides of said gap for reducing the amount ofmicrowave frequency energy radiated from said gap.

3. A device according to claim 2, wherein said choke means comprisesrespective flanges connected to said waveguide at the ends defining saidgap, at least one of said flanges including a groove for substantiallyreducing the impedance across said gap.

4. A device according to claim 3, wherein the temperature of said seconddevice is intended to be less than the temperature of said first device,and wherein the flange connected to said second waveguide sectioncontains said groove.

5. A device according to claim 3, further including means for absorbingmicrowave energy radiated from said gap.

6. A device according to claim 5, wherein said absorbing means comprisesan energy absorbent material spaced peripherally around said gap.

References Cited UNITED STATES PATENTS 2,451,876 10/1948 Salisbury.2,627,571 2/1953 Hiehle et al. 219-1055 2,832,045 4/ 1958 Sharpless.

HERMAN KARL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner.

1. A DEVICE FOR COUPLING MICROWAVE ENERGY BETWEEN A FIRST DEVICE AT ONETEMPERATURE AND A SECOND DEVICE AT A TEMPERATURE SUBSTANTIALLY DIFFERENTFROM SAID ONE TEMPERATURE, COMPRISING MEANS FOR THERMALLY INSULATINGSAID SECOND DEVICE FROM SAID FIRST DEVICE, A FIRST WAVEGUIDE SECTION INA THERMALLY CONDUCTIVE RELATIONSHIP WITH SAID FIRST DEVICE, A SECONDWAVEGUIDE SECTION ADAPTED TO COUPLE ENERGY TO OR FROM SAID SECONDDEVICE, SAID FIRST AND SECOND WAVEGUIDE SECTIONS BEING SPACED APART SOAS TO INTERRUPT THE HEAT CONDUCTION PATH THEREBETWEEN, AND CHOKE MEANSPOSITIONED AT THE GAP BETWEEN SAID WAVEGUIDE SECTIONS FOR REDUCING THEAMOUNT OF MICROWAVE ENERGY RADIATED FROM SAID GAP.